For defence services to provide effective capability, it is important that enough suitably qualified personnel are available at the right times. Essential to this, is a workforce structure that sustainably meets the personnel requirement. Specifically, an adequate number of positions in the different ranks is needed, as well as sufficient promotions and recruitment to sustain those positions over time, given the constraints on the workforce. In this paper, we mathematically explore the relationship between these parameters, describe how to determine whether a workforce structure is sustainable or not, and detail what changes may need to be made to achieve a healthy structure.Here, sustainability means that the requirement profile -the number of positions in each rank at each time -can be achieved without excesses or shortfalls of personnel. We critique a simple linear programming (LP) optimisation formulation that was applied to Royal Australian Navy (RAN) test problems, and the unexpected results that it produced. In particular, it resulted in large oversupplies in lower ranks early in a testing period of 20 years. By using a cost function that penalised oversupply and undersupply with different weightings, we found that such oversupplies were necessary in order to avoid significant shortfalls of personnel in higher ranks and later years. This provided the motivation for the main contribution of this paper: an exploration of why the oversupply was necessary and equations that could assist workforce planners to quantify the magnitude of this oversupply. This involved assessing how specific factors play a role in the sustainability of workforce structures. Backpropagation of required numbers of personnel through the ranks provided the necessary number of promotions at each rank and time in order to achieve that requirement. We refer to this number of promotions as the Required Production Number (RPN). An analysis of the distribution of time in rank permits calculation of how many promotion-eligible personnel could be produced at each rank and time. We refer to this number of producible personnel as the Suppliable Production Number (SPN).When the RPN exceeds the SPN, one of two functionally equivalent situations has to happen to avoid future shortfalls in other ranks as predicted by the model: oversupplying a rank or increasing the requirement of a rank. We quantify the extent to which the requirement needs to be increased. Conversely, when the RPN is less than the SPN, the requirement can be met without excesses or shortfalls of personnel, but personnel will spend longer in a rank before promotion. Calculations to quantify the expected time to promotion are also described. This is an important consideration as personnel waiting too long for promotion may result in decreased morale and an increase in personnel leaving the workforce.These simple and practical measures allow workforce planners to draw insights into their workforce structure. In particular, they can determine whether and why the requirement in particu...